Over-coupled resonator for broadband surface enhanced infrared absorption (SEIRA)

Detection of molecules is a key issue for many applications. Surface enhanced infrared absorption (SEIRA) uses arrays of resonant nanoantennas with good quality factors which can be used to locally enhance the illumination of molecules. The technique has proved to be an effective tool to detect small amount of material. However, nanoresonators can detect molecules on a narrow bandwidth so that a set of resonators is necessary to identify a molecule fingerprint. Here, we introduce an alternative paradigm and use low quality factor resonators with large radiative losses (over-coupled resonators). The bandwidth enables to detect all absorption lines between 5 and 10 μm, reproducing the molecular absorption spectrum. Counterintuitively, despite a lower quality factor, the system sensitivity is improved and we report a reflectivity variation as large as one percent per nanometer of molecular layer of PMMA. This paves the way to specific identification of molecules. We illustrate the potential of the technique with the detection of the explosive precursor 2,4-dinitrotoluene (DNT). There is a fair agreement with electromagnetic simulations and we also introduce an analytic model of the SEIRA signal obtained in the over-coupling regime.

In this work, the authors describe a plasmonic system composed of a dielectric cavity where one mirror is replaced with an array of metallic wires and apply it for surface-enhanced infrared absorption spectroscopy (SEIRA). The notable features of the authors' design are that it is operated in the overcoupled light-matter interaction regime and functions over a relatively broad spectral range. Simple SEIRA demonstrations based on thick layers of PMMA and ODT molecules are likewise presented.
Even though the proposed approach is described clearly and appears (in principle) useful for SEIRA applications, the manuscript fails to demonstrate significant advances over the current state of the art. My main concerns are with (1) the claimed sensitivity advances over previously published geometries, (2) the usefulness of the broadband operation, and (3) the lack of a compelling SEIRA demonstration with a relevant molecular system. Furthermore, the mechanism of the enhancement is not analyzed in sufficient detail and some data necessary for following the authors' arguments is missing. These concerns preclude publication of the manuscript in Nature Communications, as explained in more detail below.
As the authors state themselves, the different coupling regimes of plasmonic SEIRA have been extensively studied in the past 10 years, starting with the Adato et al. papers mentioned in the text. Since then, the field has progressed and many demonstrations of specially engineered plasmonic antennas have been shown for the SEIRA-based detection of ultralow quantities of molecules (see, e.g., ACS Photonics 5, 4117-4124 (2018)). These antennas still exhibit relatively broad resonances, which can resolve many different types of biomolecules simultaneously (e.g., Adv. Mater. 33, 2006054 (2021)). It is not clear how the authors' design improves over these structures in terms of sensitivity. The molecular sensing demonstrations (PMMA, ODT) are done with extremely thick layers of molecules (50-100 nm), which greatly exaggerates the sensor response in the spectra. From the performed calculations and experiments, it is nearly impossible to draw a comparison to realistic molecular sensing experiments (such as on low concentrations of molecules or ultrathin layers). This issue becomes especially clear in the comparison table in Figure S2. The authors show a reflectance modulation of "10-70%" for their own design and a PMMA thickness of 100nm. This is objectively worse than the Adato 2013 demonstration, where a PMMA layer of only 8 nm thickness produced a modulation of 25%.
Likewise, broadband SEIRA operation with high sensitivity has been conclusively demonstrated in a recent paper using wavelength-multiplexed plasmonic hook nanoantennas (Nature Communications 13, 3859 (2022)), where "the wavelength-multiplexed HNAs serve as ultrasensitive vibrational probes in a continuous ultra-broadband region (wavelengths from 6"8/ 51 $"8/"#7 %,* 26'.-4,*) 2&2*3 -0 +&(5 goes much beyond the present work by demonstrating molecular recognition from different alcohol mixtures using sophisticated machine learning algorithms. As an alternative, many recent SEIRA demonstrations employ a multiplexed approach (for example through arrays of sensor elements), which can also produce broadband operation and high sensitivity.
Finally, the authors omit some information that would be crucial for better understanding the enhancement mechanism of their design. For example, only the critically coupled and overcoupled regimes are shown. Many other approaches operate in the undercoupled regime. How those the authors' design perform there? What is the general mechanism of the broadband operation? Why is the electromagnetic near-field not maximal for the critical coupling case (as expected from theory and literature)? What are the mechanisms and limits of the spectral operating range?
In summary, I believe that the manuscript does not demonstrate a significant advance for plasmonic SEIRA (especially with the lack of a compelling sensing demonstration) and therefore recommend its rejection.
Reviewer #2 (Remarks to the Author): Paggi et al. investigates broadband, high-contrast SEIRA sensing of thin films in a previously overlooked 'over-coupled' regime. In the majority of SEIRA studies reported to date, as the authors correctly state, a common design approach has been to enhance IR absorption in bare resonator/antenna structures and then to insert target analytes in and around optical hotspots. The authors approach is different in that they utilized an over-coupled resonator structure consisting of an array of gold ribbons coupled to a gold mirror. While the reflectivity spectrum of the bare resonator shows weak IR absorption, interestingly the structure shows broadband SEIRA signals from deposited PMMA and DNT thin films. This counter-intuitive outcome is nicely explained by using analytical equations and computer simulations.
The manuscript is clearly written and suggests a promising new approach for the SEIRA sensing community, so I recommend publication in Nature Communications after the authors address the following comments: 1. Before delving into the discussion on the critical vs. over-coupled regimes, it'd be nice to add more materials to explain the 'mechanics' of their resonator structure and how the geometrical parameters (gap width, ZnS thickness, ribbons periodicity, etc.) influence the overall IR absorption and transition from under-to critical-and over-coupled regimes.
The basic geometry of their resonator structure is reminiscent of a patch antenna structure. Also, similar motifs have been used by other researchers (albeit mostly in critically coupled regime), for example, the gapped gold antennas on a reflector [L. Dong et al. Nano Lett. 2017, 17, 5768-5774] and reflector-coupled gold ribbon arrays separated by a mid-IR transparent cavity [I.-H. Lee et al. Nature Nanotechnology 2019, 14, 313]. Thus, it would be nice to present the authors' design in the context of such relevant previous works.
2. If it makes sense to improve the flow, the author might consider moving the field enhancement simulation ( Figure S1) into the main text.
3. Figure 3 PMMA results: -Labels 'a', 'b', 'c', 'd' are mentioned in the caption but are missing in the figure. Please add proper labels.
-In (a) schematic, the authors depict ideal coverage of 45-nm thick PMMA on the film and inside the gap, which is unrealistic. Has the authors considered non-uniform coverage of the structure after PMMA coating? Could there be more (thicker) PMMA in and around the slit area? -More experimental spectra could be shown in Figure 3. 4. Likewise, when the authors discuss the SEIRA results obtained from DNT in Fig. 4, I was hoping to find more details on how they prepared and characterized the sample after depositing a droplet. In the Supplementary Information, the authors infer that ~100 nm thick DNT film should have been formed, but more independent measurement would be desirable. Was the coverage very uniform across the active surface area? 5. Could the authors discuss strategies to further optimize the performance? Their simulations show the E-field intensity enhancement (|E|^2) of 300x in their structure. By shrinking the gap width, it may be possible to further enhance the field enhancement factor. Could that lead to improved SEIRA sensitivity/contrast in the over-coupled regime? 6. In their 'Conclusion', it'd be nice to go beyond their current structure and discuss if the strategy of performing SEIRA in the over-coupled regime can be generalized. For example, could the authors suggest general design rules for other SEIRA structures (e.g. nanorods) based on what they've learned here? 7. In Figure 4, reflectivity different (y-axis), is the unit supposed to be %?
First, we want to thank both reviewers for their detailed reading of our manuscript and for the questions and remarks that has helped us to significantly improve the quality of our manuscript, which we have now complemented both in the main manuscript and the supplemental material after completing further studies.
Please, find below the details of our response to their comments and the actions which we have taken to modify our manuscript accordingly.

Reviewer 1
General comment: "In this work, the authors describe a plasmonic system composed of a dielectric cavity where one mirror is replaced with an array of metallic wires and apply it for surface-enhanced infrared absorption spectroscopy (SEIRA). The notable features of the authors' design are that it is operated in the overcoupled light-matter interaction regime and functions over a relatively broad spectral range. Simple SEIRA demonstrations based on thick layers of PMMA and ODT molecules are likewise presented.
Even though the proposed approach is described clearly and appears (in principle) useful for SEIRA applications, the manuscript fails to demonstrate significant advances over the current state of the art. My main concerns are with (1) the claimed sensitivity advances over previously published geometries, (2) the usefulness of the broadband operation, and (3) the lack of a compelling SEIRA demonstration with a relevant molecular system. Furthermore, the mechanism of the enhancement is not analyzed in sufficient detail and some data necessary for following the authors' arguments is missing. These concerns preclude publication of the manuscript in Nature Communications, as explained in more detail below." Response: We reply below to the detailed concerns of the reviewer. In particular, we have introduced in our revised manuscript new results on more relevant molecular systems (thinnest layers: 8 nm for PMMA and 20 nm for DNT) and developed the mechanism of the enhancement thanks to a model based on the temporal coupled mode theory.

Comment 1 :
(1) the claimed sensitivity advances over previously published geometries As the authors state themselves, the different coupling regimes of plasmonic SEIRA have been extensively studied in the past 10 years, starting with the Adato et al. papers mentioned in the text. Since then, the field has progressed and many demonstrations of specially engineered plasmonic antennas have been shown for the SEIRA-based detection of ultralow quantities of molecules (see, e.g., ACS Photonics 5, 4117[4124 (2018)). These antennas still exhibit relatively broad resonances, which can resolve many different types of biomolecules simultaneously (e.g., Adv. Mater. 33, 2006054 (2021)).
*H =G BCH 7@95F <CK H<9 5IH<CFGU 89G=;B =ADFCJ9G CJ9F H<9G9 GHFI7HIF9G =n terms of sensitivity. The molecular sensing demonstrations (PMMA, ODT) are done with extremely thick layers of molecules (50-100 nm), which greatly exaggerates the sensor response in the spectra. From the performed calculations and experiments, it is nearly impossible to draw a comparison to realistic molecular sensing experiments (such as on low concentrations of molecules or ultrathin layers). This issue becomes especially clear in the comparison table in Figure S2. The authors show a reflectance modulaH=CB C: Ved-kdpW :CF H<9=F CKB 89G=;B 5B8 5 /,," H<=7?B9GG C: eddBAT 2<=G =G C6>97H=J9@M worse than the Adato 2013 demonstration, where a PMMA layer of only 8 nm thickness produced a modulation of 25%.
Response: We thank the reviewer for the additional references and we have added them to our revised manuscript.
The reviewer has pointed out an important issue regarding the fairness of the comparison between geometries. The comparison between systems that are demonstrated with different molecules is complex as the strength of the molecules absorption is paramount to the response of the complete system resonator + molecules. And this is what motivated our choice for PMMA, since as shown in the comparison table in supplemental material, SEIRA has been demonstrated on many geometries with this molecule with thicknesses ranging from 8 to 100 nm.
As requested, in our revised manuscript, we have made new experiments to demonstrate the sensitivity for thinner layers between 8 to 20 nm for PMMA and for 20 nm for DNT.
Besides, the sensitivity of the resonator can be improved by shrinking the size of the slit, as is demonstrated numerically in the following figure (added as Fig. S3). By reducing the dimensions of the slits to 10 nm by 8 nm (width x height), one can obtain a reflectivity difference up to 50 % for the 1730 cm-1 PMMA mode when the slit is filled by the analyte.
In the original manuscript, we show experimentally a reflectance modulation of 10-70% on a PMMA thickness of 45 nm and we have also introduced the responsivity of the resonator as being in the order of 1-3%/nm with numerical simulations. This gives a value of 24 % for an 8 nm thick layer, which seems comparable to the value we have indicated for the Adato 2013 demonstration.
But this value of 25% indicated for the Adato paper is due to both the SEIRA effect and the SPR (surface plasmon resonance) effect. may be confusing as on one hand it should have been underlined that it is obtained with calculated spectra, the experimental reflectivity difference value is lower, on another hand it is not clear that the reflectivity difference is induced by the enhancement of the mode only and not impacted by the shift of the resonance. The following figure shows the superposition of the Adato spectrum and the signature of PMMA, and underlines the complexity recovering the seira signal.
It must be emphasized that, for our design, vibrational modes of PMMA at higher wavelength are still visible, the broadband effect is preserved for lower slit volume.
-New results on thinner layers of PMMA and DNT have been added  -Electromagnetic computations for SEIRA on a smaller slit has been introduced ( Fig. S3 and Sec. 1.4 of Supp. Mat.) Comment 2 : (2) the usefulness of the broadband operation Likewise, broadband SEIRA operation with high sensitivity has been conclusively demonstrated in a recent paper using wavelength-multiplexed plasmonic hook nanoantennas (Nature Communications egR glim \fdff]]R K<9F9 VH<9 K5J9@9B;H<-multiplexed HNAs serve as ultrasensitive vibrational probes in a continuous ultra-6FC5865B8 F9;=CB \K5J9@9B;H<G :FCA j OA HC m OA]TW 2<9 DI6@=G<98 D5D9F =B fact goes much beyond the present work by demonstrating molecular recognition from different alcohol mixtures using sophisticated machine learning algorithms. As an alternative, many recent SEIRA demonstrations employ a multiplexed approach (for example through arrays of sensor elements), which can also produce broadband operation and high sensitivity.
Response: The authors agree with the referee that the mentioned articles must be included as citations to complete the state of the art in broadband seira. The accomplished multiplexing approach, as stated, can lead to 3 µm wide enhancement range. The structure presented in our manuscript has the advantage of enhancing over a 5 µm wide wavelength range with only one resonator. Additionally, our signal reproduces the molecular absorption spectrum and is free of the SPR effect. In both cases, the system can operate with a single pixel detector. The third alternative discussed by the reviewer is to use arrays of sensors with multiplexed approach, and this can be done on an even broader range. This third approach may address other needs, but is not directly comparable to the two first.
We believe that our system can also present advantages in given situations compared to the Nature Comms 13, 3859 (2022) paper. Juxtaposed systems are probing the response of different sets of molecules, while in our case, the same molecules are probed. This could be interesting when studying dynamic behaviors of molecules, in particular for biological molecules (e.g., following DNA or proteins interactions).

Action taken by the authors:
-We have added the reference Nature Communications 13, 3859 (2022) given by the reviewer on the multiplexed approach, as well as one reference on the multiplex approach with a sensor array in the introduction of our revised manuscript.
Comment 3: (3) the lack of a compelling SEIRA demonstration with a relevant molecular system Response: As suggested by the reviewer, we have included in our revised manuscript new experimental results on PMMA and DNT. On PMMA, we have made various thicknesses ranging from 8 nm to 20 nm (the thickness of our resonator).
2,4-dinitrotoluene is an explosive derivative, that was chosen to offer a practical demonstration. It it a volatile compound and we have made further measurements for a 20 nm layer.

Action taken by the authors:
-We have added results to the revised manuscript with more relevant systems (see Figs. 4 and 5). Also, the results on DNT are now done with the same resonator materials as with PMMA (ZnS instead of SiO 2 ).
Comment 4: Furthermore, the mechanism of the enhancement is not analyzed in sufficient detail and GCA9 85H5 B979GG5FM :CF :C@@CK=B; H<9 5IH<CFGU 5F;IA9BHG =G A=GG=B;T Finally, the authors omit some information that would be crucial for better understanding the enhancement mechanism of their design. For example, only the critically coupled and overcoupled regimes are shown. Many other approaches operate in the undercoupled regime. How those the 5IH<CFGU 89G=;B D9F:CFA H<9F9Q 3<5H =G H<9 ;9B9F5@ A97<5B=GA C: H<9 6FC586and operation?
What are the mechanisms and limits of the spectral operating range? Reponse: We agree with the referee that a more general discussion about the mechanism of the broadband operation would improve the understanding of the experimental results presented in this manuscript. Hence, we used the Temporal Coupled Mode Theory to derive an analytical expression of the reflection coefficient in presence of an absorbing line (equation 2 of the main text) that enables the study of the variation of the amplitude of the SEIRA signal as a function of the different parameters of the system.
In particular we show that for a good metal, i.e., ,nr<<( -r--b): the radiative decay rate ,r dictates the operating bandwidth of our method. Indeed, the frequency response of the absorbing material has to overlap with the frequency response of the resonator to be detected. From this observation, we directly know that under-coupled, and critically coupled resonators would operate very badly in the broadband regime. the SEIRA signal is maximal when the radiative decay rates: ,r= (-r--b). This demonstrates that the SEIRA signal is maximal for a resonator highly over-coupled (+ 2 / 1 / 01 >>1).
Eventually, to simplify this discussion, we provide an intuitive picture of the mechanism behind the enhancement of the SEIRA signal. We show that the absorber induces a modification of the frequency -r= -r+ -& and of the nonradiative decay rate ,nr= ,nr+,& of the resonator that brings it back towards resonance and critical coupling.
Actions taken by the authors: -We added a complete section in the Supplementary Information (Section 3) that contains 3 figures and the analytical study of the reflection coefficient in presence of the absorber using temporal coupled mode theory, and demonstrate that the SEIRA signal is maximal for a resonator highly over-coupled + * (% We added a paragraph in the revised manuscript with equations 1 and 2 and added Fig. 2 to summarize the result of the derivation. Comment 5: Why is the electromagnetic near-field not maximal for the critical coupling case (as expected from theory and literature)?
Response: The definition of critical coupling (R=0) is not equivalent to maximum electromagnetic near field. Critical coupling entails a large enhancement for metals that are almost lossless. Here, as we show in Fig. S2, the maximum electromagnetic near field is obtained in the over-coupling regime (f = 3) when the slit geometry (h s , w s ) is kept constant as well as the resonance wavelength.
Action taken in the manuscript: -We have added a discussion in Sec. 1.3 of the supplemental material.

Reviewer 2 :
General comment: "Paggi et al. investigates broadband, high-contrast SEIRA sensing of thin films in a previously overlooked 'over-coupled' regime.
In the majority of SEIRA studies reported to date, as the authors correctly state, a common design approach has been to enhance IR absorption in bare resonator/antenna structures and then to insert target analytes in and around optical hotspots.
The authors approach is different in that they utilized an over-coupled resonator structure consisting of an array of gold ribbons coupled to a gold mirror. While the reflectivity spectrum of the bare resonator shows weak IR absorption, interestingly the structure shows broadband SEIRA signals from deposited PMMA and DNT thin films. This counter-intuitive outcome is nicely explained by using analytical equations and computer simulations.
The manuscript is clearly written and suggests a promising new approach for the SEIRA sensing community, so I recommend publication in Nature Communications after the authors address the following comments:" Response: We are pleased that the referee finds our work timely and of interest for Nature Communication's readership. We are grateful for the positive evaluation on the scientific soundness and accessibility of our manuscript. We acknowledge the referee's constructive remarks which we address in detail below.
Comment 1: Before delving into the discussion on the critical vs. over-coupled regimes, it'd be nice to add more materials to explain the 'mechanics' of their resonator structure and how the geometrical parameters (gap width, ZnS thickness, ribbons periodicity, etc.) influence the overall IR absorption and transition from under-to critical-and over-coupled regimes.
The basic geometry of their resonator structure is reminiscent of a patch antenna structure. Also, similar motifs have been used by other researchers (albeit mostly in critically coupled regime), for example, the gapped gold antennas on a reflector [L. Dong et al. Nano Lett. 2017, 17, 5768-5774] and reflector-coupled gold ribbon arrays separated by a mid-IR transparent cavity [I.-H. Lee et al. Nature Nanotechnology 2019, 14, 313]. Thus, it would be nice to present the authors' design in the context of such relevant previous works.
Response: We thank the referee for pointing out the lack of this discussion on the resonator itself. We have added a discussion and a parametric study into the supplemental material (Sec. 1.2) to give insights of the behavior of the resonator. The resonance mechanism is pretty different from the two antennas of the publications given by the referee, even if they are relying on metal-insulator-metal stacks. In Dong et al. paper, the resonator acts as a bow-tie, and in Lee et al. paper, the resonator is a gap-plasmon cavity with a graphene layer. We provide a comparison of the behavior of the two configurations of optical Helmholtz resonators in the following figure (Fig. S2). We must emphasize that one important property of this resonator is to concentrate the electric field in the slit, while the magnetic field is concentrated in the dielectric layer.
Action taken by the authors: -We have added in the supplemental material detailed information on the mechanism of resonance (Sec. 1.2 of the sup. Mat and Fig. S2).
Comment 2: If it makes sense to improve the flow, the author might consider moving the field enhancement simulation ( Figure S1) into the main text.
Response: We have followed the suggestion of the reviewer in our revised manuscript Action taken by the authors: -The field enhancement figure is now included in the Fig. 1 of the manuscript. Response: We thank the referee for pointing out this. We have corrected this in our revised manuscript.
Comment 3-ii: -In (a) schematic, the authors depict ideal coverage of 45-nm thick PMMA on the film and inside the gap, which is unrealistic. Has the authors considered non-uniform coverage of the structure after PMMA coating? Could there be more (thicker) PMMA in and around the slit area?
Response: We do agree that the coverage of PMMA depicted is ideal. Usually, a spin-coated resist topography is influenced by the relief of lower layers.
We have done the following to try to assess the uniformity of the PMMA on the resonator: -ellipsometric measurement of the thickness of PMMA near the resonators.
-AFM measurements on 3 periods of the resonator (1x1 µm² scanned area) at different locations and for 2 PMMA thicknesses.
-Infrared measurements on different spots (50 µm diameter) to assess the homogeneity of the infrared response.
We have added error bars in Fig. 4 of the main manuscript, we think that the homogeneity issue raised by the referee is partially responsible for the observed dispersion of the measurements. The infrared measurement s are averaging the results on nearly 150 periods but there are still variations on the surface (the thickness of a spin-coated layer tends to be thicker near the edges of the sample). Ellipsometric measurements have shown that we typically had a 2 nm variation on the same sample.
Action taken by the authors: -Sec. 2 of the supplemental material has been added with Figs. S3 and S4.
-Error bars on Fig. 4 (that now includes experimental results) account for the variations of homogeneity.
Comment 3-iii: -More experimental spectra could be shown in Figure 3.
Response: We have added more experimental data points for various thicknesses of PMMA and DNT (see Figs. 4 and 5). See also our response to the next comment for the DNT measurements.
Action taken by the authors: -Figs. 4 and 5 of the revised manuscript include more experimental data (as well as direct comparison with electromagnetic computations).
Comment 4: Likewise, when the authors discuss the SEIRA results obtained from DNT in Fig. 4, I was hoping to find more details on how they prepared and characterized the sample after depositing a droplet. In the Supplementary Information, the authors infer that ~100 nm thick DNT film should have been formed, but more independent measurement would be desirable. Was the coverage very uniform across the active surface area?
Response: We have given more details on the preparation and characterization of the sample. In fact, DNT is a volatile compound and very thin layers evaporate in a couple of minutes. This makes it hard to provide independent characterization of the thickness of DNT as could be done in the case of PMMA. In our revised manuscript, we have introduced additional measurements for two distinct times (one minute apart) and with a better control of the original deposited volume thanks to a microplotter device.
First, the microplotter allows us to deposit a controlled volume of a diluted DNT in acetonitrile on a controlled area of 500 x 500 µm² (with a typical lateral resolution of 10 µm). The volume deposited is measured by the difference in the filling level of the capillary measured by a camera. The acetonitrile is evaporated in typically one minute, and we have been able to make infrared measurements at 3 and 4 minutes. One minute later, all the DNT is evaporated and there is no signal remaining.